Chair back



FIG. 1 is a perspective view of a chair back, sowing the new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is left-side elevation view thereof;

FIG. 5 is a right-side elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof. 

CLAIM The ornamental design for a chair back, as shown and described. 